A typical tube-and-shell steam generator being used in nuclear steam generators has a vertical vessel which houses a plurality of U-shaped tubes. Heating fluid enters one leg (hot leg) of these tubes, and leaves via the other leg (cold leg), giving up heat in the process to vaporize a fluid flowing on the outside of the tubes. A deck or plate is located above the tubes, on which a plurality of water-steam separators are located. These separators remove the water from the steam-water mixture, returning the water to the bottom of the vessel through a downcomer.
Because the heating fluid is hotter in the hot leg than the cold leg of the U-tubes, more steam is generated on the hot leg side of the steam generator than the cold leg side. This maldistribution causes overloading of the separators on the hot leg side. The steam flow rate is maximum at the extreme of the hot leg region. This can cause excessive moisture carryover with the steam exiting from the separators.